1. Technical Field
The present invention relates to an atomic oscillator and a manufacturing method of the atomic oscillator.
2. Related Art
In recent years, an information equipment, such as a cellular phone or a smartphone, is improved to attain high performance and multi-functionality, and a request for frequency accuracy and frequency stability of an oscillator used for data communication becomes more and more severe. As the oscillator to satisfy the high request, an atomic oscillator using a resonance phenomenon of an atom such as rubidium or cesium is widely used.
As the atomic oscillator, there is known one using an EIT (Electromagnetically Induced Transparency) phenomenon which occurs when two kinds of resonant lights whose frequency difference is coincident with the resonance frequency are simultaneously irradiated to the atom or one using an optical micro double resonance phenomenon which occurs when exciting light is irradiated to the atom and a microwave coincident with the resonance frequency of the atom is applied to a cavity resonator. In both the cases, the oscillation frequency of a voltage controlled oscillator is controlled according to the intensity of light passing through an atomic cell, and the oscillation signal is frequency-converted, so that the signal of the resonance frequency of the atom is obtained.
Here, for example, when an oscillation signal of a voltage controlled oscillator is desired to be made an accurately desired frequency (for example, 10 MHz) as an output signal of an atomic oscillator, since the resonance frequency of a rubidium atom is 6.8346826128 GHz, and the resonance frequency of a cesium atom is 9.192631770 GHz, the resonance frequency of the atom is not an integer times larger than the oscillation frequency of the voltage controlled oscillator.
In the invention of JP-A-2001-36405 (Patent Literature 1), a desired resonance frequency signal can be obtained without using an LC tank circuit using a coil and a capacitor or a passive multiplication circuit such as a varactor diode. Accordingly, it is unnecessary to perform adjustment such as impedance matching between a low-order multiplier and the atomic oscillator, and it is also unnecessary to generate the resonance frequency of the rubidium atom by using amplitude modulation. Accordingly, the atomic oscillator can be miniaturized, the manufacturing period can be shortened, and the manufacturing cost can be reduced.
Here, in the atomic oscillator, there is a manufacturing variation in pressure of a buffer gas such as Neon (Ne) or Argon (Ar) to be injected into an atomic cell (gas cell). Since the shift amount of the resonance frequency also varies with the manufacturing variation of the buffer gas, in order to obtain the desired frequency as the output signal, in the invention of Patent Literature 1, a direct digital synthesizer (Hereinafter referred to as DDS) is required to be provided. However, when the DDS is used, the waveform of the output signal deteriorates.